Specification is ongoing in 3GPP for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) that is the next generation of Radio Access Network (RAN). Another name for E-UTRAN, used in the present specification, is Long Term Evolution (LTE) RAN. The core network to which E-UTRAN is connected is called the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) network. Both the E-UTRAN and the EPC (and possibly some other node(s), such as the Home Subscriber Server (HSS), depending on the definition of the EPC) comprise together the Evolved Packet System (EPS), which is also known as the SAE/LTE network. A base station in this concept is called an Evolved NodeB (eNodeB or eNB).
As originally envisaged, an eNB would be available to all subscribers of the Public Land Mobile Network (PLMN) of which the eNB is a part, and to subscribers of other network having appropriate roaming agreements with this PLMN. However, ongoing studies also include the possibility to have an E-UTRAN base station which provides home or small area coverage for a limited number of users. This base station is, in 3GPP and in this document, called a Home Evolved NodeB (HeNB) or home base station. Other names used for this type of base station are LTE Home Access Point (LTE HAP) or LTE Femto Access Point (LTE FAP). In 3G Universal Mobile Telecommunication System (UMTS) the equivalent base station to an HeNB is referred to as a Home Node B (HNB). While HeNBs are used herein, similar concepts apply to the HNB of the 3G UMTS systems.
An HeNB typically provides regular services for the end users and can be connected to the mobile core network using an IP-based transmission link. The radio service coverage provided by an HeNB is called a femtocell in this application. Furthermore, a femtocell is normally a Closed Subscriber Group (CSG) cell, i.e., a cell in which only a limited but variable set of users is normally allowed to access the network. The HeNB would, in most cases, use the end user's already existing broadband connection (e.g. xDSL and Cable) to achieve connectivity to the operator's Public
Land Mobile Network (PLMN) and possibly to other eNBs/HeNBs, via some gateway, e.g. a HeNB gateway. A reason for providing wireless local access using HeNBs and femtocells is to provide cheaper calls or transaction rates/charges when a device (e.g., a mobile phone) is connected via an HeNB as compared to when that device is connected via an eNB. The proposed architecture also allows for increased traffic levels and user density.
Certain HeNBs may allow access to all subscribers having access to the PLMN, whilst prioritising subscribers belonging to the associated CSG. Such HeNBs are sometimes referred to as “hybrid” HeNBs. Other HeNBs may provide open and non-prioritised access to all subscribers and these are referred to as “open” HeNBs.
More generally, an HeNB and similar devices can be considered to be a sort of “home base station”. As used herein, the term “home” is used to modify the phrase “base station” to distinguish such equipment from other conventional base stations based upon characteristics such as one or more of: (1) geographic radio coverage provided (i.e., home base station coverage area is normally less than “regular” base station coverage area), (2) subscriber access (i.e., the subscribers who can obtain service from the home base station may be limited whereas a “regular” base station will typically provide access to any subscribers (or at least to a larger group of subscribers than a home base station) who are within range), and (3) home base stations are normally installed by the end users themselves without any intervention from the operator's personnel, whereas regular base stations are typically installed by operator personnel. This latter quality of home base stations suggests that the installation will generally be highly automated and of a “plug and play” nature. Note, however, that home base stations need not literally be installed in personal residences, and may find applications in businesses, public areas, etc., wherein the qualities of a home base station are desirable to, e.g., supplement coverage provided by regular base stations. Home gateways, as the phrase is used herein, are gateways which interface home base stations with a node in the radio communication system, e.g., a core network node.
For a given Public Land Mobile Network (PLMN) (to which both the HeNB and the CSG members are assumed to belong), the CSG data is managed by the Operation Maintenance Administration and Provisioning (OMA&P) system (sometimes also known as OAM&P system) of the PLMN to which the HeNB connects. The OMA&P system deals with, for example, configuration, supervision and tuning of the radiocommunication network, administration of subscriber related data and provisioning of data, features and services. The CSG “whitelist” of a subscriber, which includes identities of the CSGs the subscriber is a member of and thus is allowed to access, is managed by the OMA&P system and the HSS of the subscriber's home PLMN and the subscriber's UE (e.g., its USIM).
Up to and including 3GPP E-UTRAN Release-9, handover of a UE between eNBs (including eNBs per se and HeNBs) was conducted over the S1 vertical interface between the eNBs and the Mobility Management Entity (MME). Release-10 [E-UTRA and E-UTRAN Overall Description; Stage 2 (Release 10); 3GPP TS 36.300 Rel-10 V10.4.0 (2011-06)] introduced the possibility of conducting the handover over the X2, peer-to-peer, interface between eNBs, but only for those cases where access control at the core network is not needed, i.e. when source and target belong to the same CSG or when the target is an open HeNB.
3GPP RAN3 has recently started work on a Release-11 Study Item [Alcatel-Lucent, Proposed SID: Further enhancements for HNB and HeNB; 3GPP RAN RP-110456] and which is concerned with enhanced mobility between eNBs and HeNBs, aiming among other things to enable X2-based handover also in cases where access control is required (e.g. between HeNBs belonging to different CSGs or when eNBs are involved). In particular, enhanced mobility scenarios between macro and hybrid/open HeNBs (see FIG. 1) have been recently prioritized by RAN3 in the scope of this SI [Sec. 5.2.1 of Mobility Enhancements for H(e)NB (Release 11); 3GPP TR 37.803 V0.1.2 (2011-06)]
It is now required to address the issue of how to aid the source eNB in the selection of the most appropriate X2 handover candidate for a CSG UE, while minimizing the need for core network involvement for access control and minimizing the risk of handing over to a wrong or inappropriate cell (which may result in a handover failure).